Window temperature control



June 1, 1954 WITNESSES:

INVENTORS Clarence L.Mershon and BY George W.Nogel.

ATTORNEY Patented June 1, 1954 UNITED STATEh ZPATENT OFFICE Nagel, Pittsburgh, Pa house Electric Corpora Pa., a corporation of Fe assignors to Westingtion, East Pittsburgh, nnsylvania Application March 17, 1950, Serial No. 150,266

4 Claims.

This invention relates to electronic controls especially for automatic temperature regulation, and particularly to a system for controlling the supply of electrical energy for the heating of a body to m intain it at a desired temperature.

In general, the invention contemplates the provision of an electronic control for use in con nectlon with electrically heated windows, such aircraft windows or the like, and in which an electrical heating element is provided in heat exchanging relationship with a windowpane. In order to govern the supply of electrical energy to such heating element to maintain the window temperature at a desired value (for example to prevent the accumulation of ice thereon), it has been proposed to provide a temperature-sensing element also in heat exchange relation with the windowpane, and to control the supply of electrical power to the heating element in accordance with the existing temperature of the windowpane as sensed by the sensing element. In prior copending applications of Pridmore and Nagel, Serial No. 9858i, filed June 11, 1949, and Mershon, Serial No. 9,860, filed June 11, 1949, now abandoned, both assigned to the owner of the present application, a window heating and control system has been disclosed which is especially designed to discontinue the flow of heating energy to the windowpane upon various kinds of failure which may occur in the sensing element itself or in the electric circuit which controls the heater. These prior arrangements, for example, provide for discontinuation of the heating energy now upon either short circuit or open circuit condition in the sensing element, as well as upon failure of the supply voltage to the control circuit and upon failure of the vacuum tube or tubes therein.

The actual operation of turning on and 01f the supply of electricity to the window heating element, in the above-mentioned applications as well as in numerous other systems of this gen eral type, is accomplished by means of an electric relay whose contacts are opened and closed to open and close the supply circuit to the heating element. In all such prior arrange ments of which we are aware, however, no provision has been made for the interruption of the supply of heating energy to the window heater in the event of the sticking of the relay contacts in circuit-closed condition. Since the sticking of relay contacts is of fairly common occurrence, and since the result of such sticking in systems of the type under consideration would usually result in serious overheating of the windowpane, it is highly desirable to provide the apparatus with means for interrupting the supply of heating energy before the heater itself or parts associated therewith are damaged.

It is accordingly a principal object of this invention to provide a, window heating control with means for discontinuing the supply of electric energy to the heater element upon the occurrence of a sticking of the relay contacts.

It is a further object of the invention to pro vide a system of this type which will not interfere with the normal operation of the window temperature control; that is, one in which the supply of heating energy is interrupted by operation of the safety device only in response to a condition of the apparatus corresponding to a window temperature somewhat higher than that which can be attained by the window during normal operation of the electrical control circuit.

Still another object of the invention is to provide a safety arrangement of the above type in which the condition of sticking of relay contacts will not merely permanently interrupt the supply of heating energy to the window heater, but will interrupt such supply for a suflicient time to prevent damage to the window and its associated parts, and will then re-establish the supply of heating energy, this cycle being repeated indefinitely, or until the sticking contacts are released either fortuitously or by the intervention of personnel.

Still a further object of the invention is to provide a system of the above type with an alarm or indicator which will bring to the attention of opcrating personnel the fact that the device is operating with a condition of failure of contacts; that is, that the safety feature of the present in vention is in fact in operation, so that corrective measures may be instituted promptly.

Ancillary objects of the invention are to provide the features outlined above in a form suitable for economical incorporation into existing control apparatus and which can be provided at low cost during the manufacture of such controls.

The above and other objects and advantages of the invention will best be understood by referring to the following detailed specification of a presently preferred embodiment thereof, taken connection with the accompanying drawing, whose single figure is a schematic diagram of an apparatus incorporating the principles of this invention.

Referring now to the drawing, numeral It designates diagrammatically a window such as a window of an aircraft or other vehicle, the same being provided with a resistance-type electric heating element l2 which may conveniently be distributed about the area of said window (for example, by applying NESA coating to the glass surface or surfaces), which heating element is normally connected as at terminals i l to a source of electric current for heating purposes, shown as an alternating-current line, the circuit to said source including the normally opened contacts N5 of an electromagnetic relay is which is operated under the control of a temperaturesensing element 20.

Sensing element 26 may desirably comprise length of wire having a finite temperature coefficient of resistivity and arranged in heat conducting relation to the material comprising win dow it". For the purpose of averaging out local variations in the temperature of said window, the sensing element 2 3 may desirably be distributed about the area of said window, it being understood that there is no electrically conductive connection between the heating element l2 and the sensing element All.

In order to utilize the resistance of sensing element 28 to govern the application of heater power from a source i l through contacts iii to heating element 12, there is provided a Wheatstone bridge arrangement designated generally by numeral 22, one leg of which is constituted by sensing element and the electrically conducting leads therefor. The remaining legs of the bridge are formed by resistors 2 and the last two of which are joined together by a series resistor 30 provided with a movable tap 32 which may be adjusted to balance out circuit variations and to set the controlled temperature of window H) to a desired value.

A transformer S l has its primary winding connected to the power supply terminals i l (or to any equivalent alternating-current source) and is provided with a secondary winding 3'5 connected to one diagonal of bridge 22, for example, to the lower terminal of resistor 23 and to the junction between resistors 24 and 26. In accordance with well-established principles of bridge operation, any unbalance in the bridge network will be reflected in the form of a potential across the opposite diagonal of the bridge; that is, between tap 32 and the junction of resistor 2d and sensing element 2i Since the voltage supply for the bridge is an alternating voltage, the output voltage of the bridge when in unbalanced condition may conveniently be amplified by applying it to the grid of a vacuum tube amplifier Via, which may be one portion of a dual triode tube such as an RMA type IZAXT, negative grid bias being provided by a resistor-condenser combination 38, it in a well-known manner. The space current of triode unit Via is utilized to produce an amplified space current variation in triode unit Vlb by the voltage drop in load resistor a l. The resistor 42 is used to decrease the gain of the amplifier. Decreasing gain decreases the number of times the relays are actuated and prolongs relay life. Numerals M5 and ll designate smoothing condensers having the usual function.

The space current of triode unit Vib produces a variation in the potential in the control grid of the first triode unit V2a of a second dual triode vacuum tube which may be, for example, an RMA type 12AU7. In the cathode return lead of this triode V2a, there is connected the coil of an electromagnetic relay 48; normally open contacts 50 control the application of current to the operating coil of relay l8 from any convenient power source such as a direct-current source connected to terminals 52. This circuit includes also the contacts 54 of a relay 56 whose coil is in series with the cathode return circuit of the second triode section V22), these contacts 5A being shown closed in the drawing since this is their condition during normal operation of the device in the absence of any sticking of the contacts 58 of relay 48. Under this normal condition, the control of the heating current relay I8 is exercised solely by relay 48 which is operated by the amplifier system described so as to connect the window heater ill to the power supply circuit h whenever the window temperature drops below the predetermined value as sensed by sensing element 29.

The heaters for the cathodes of vacuum tubes Vi and V2 are indicated by numerals 53 and Gil, and these are connected in series with a sec ondary winding 62 of transformer 34,, as is well known in the art. in accordance with the teaching of the prior applications referred to above, cathodes of vacuum tube V2 are returned by a lead 5 and connection 58 to that side of supply line M which is not connected to the anodes of triode units Vila and V2b. This connection is not made direct, but is completed through a resistance (as here shown, the resistance of one or" the heater elements of tube Vii) which provides sufficient negative bias on the grids of the units V2a and V 22) to reduce the space currents therein nearly to zero when these grids are brought to the potential of the lower side of the supply line, as occurs if sensing element is should be short-circuited.

it will be observed that the grid of triode unit V21) is connected to a tap 68 of a biasing resistor so that for any given condition the grid of this triode unit V21) will be less negative than the grid of triode unit V211. Thus, the first section of the tube with its grid more negative than that of the second section will cut oil and deenergize relay it before relay 5% is deenergized. So long as contacts 56 are able to open to interrupt the supply of current to heater it, therefore, relay 53 will determine the maximum temperature that will be allowed in window iii. Should contacts 58 sticl; or for any reason fail to open upon the normal drop of current through relay 55 corresponding to the desired maximum window temperature, heater power from terminals ill will continue to be applied to heater i2 and the additional bridge unbalance will ultimately produce cutoii: in the second triode section V212, and the drop in current in the coil of relay will cause opening of its contacts as so that relay it will be deenergized to cut off t e supply of power to heater l2. In other words, if contacts 5% stick, control of the application of heating current is assumed by relay 5% and triode unit V 2 b, and the window heater will be cycled at a slightly higher secondary controlled tem erature established by the difference in grid bias of units Vic and V in accordance with the setting of tap on grid resistor ill. Whenever contacts 553 are restored to their operating condition, control oi; the window temperature will automatically be restored to relay 48.

In order to advise operating personnel of the fact that relay has taken over control of the heating system, and so that corrective measures may be applied to contacts 50, a signal lamp or other form of alarm may be provided, This may conveniently take the form of a high impedance lamp '52 connected across the contacts 54 so that it will be lighted in the event of an opening of those contacts 54 in response to failure of contacts E3. The impedance of this indicator lamp must be such that it will not pass sufiicient cur rent to hold relay coil 18 energized. Where the voltage 52 is twenty-four volts, this lamp should be an ordinary twenty-fourvolt filament lamp. Where the voltage is sufiicient, a neon lamp or the like is suitable for this purpose. Obviously, however, any desired type of lamp or other indicator might equally well be provided and controlled by a separate set of contacts on relay 5t.

Suitable circuit values which been found to operate satisfactorily in the novel arrangement are as follows:

Resistors:

24 ohms 300 26, 28 do 300 32 do 109 38 megohm 1 t2 do 1 3 ohms 3300 megohm 1 Capacitors:

2d mfd .01 it do .01 i8 do .01 1'6, 74 do .25 Primary voltage (14) volts 115 Secondary 62 voltage do 24 Secondary 38 voltage do 15 Capacitors l4 and It are shown as connected across the coils of relays 55 and 5%, respectively, these being chosen in accordance with Well-known .irinciples so as to smooth out variations in the currents through the coils and eliminate chattering of the contacts of these relays. The values of such capacitors, the parameters of the relay windings, and the like are to be chosen to suit the supply voltages employed in accordance with ordinary design considerations.

While the invention has been described in connection with a specific embodiment, it is to be understood that the principles thereof may be applied to apparatus of diiferent forms without departing from the scope of the invention as defined in the appended claims.

We claim as our invention:

1. In an electronic control circuit of the type employing a sensing circuit capable of producing a control potential dependent on a condition being sensed and a control circuit comprising a first discharge device having an anode, a cathode and a control grid, means for impressing a first bias of a predetermined magnitude on said control grid, and a relay having its coil in series with the discharge path of said device and a pair of contacts in series with a controlled circuit, the improvement WhlCh comprises a second discharge anode and a cathode connected those of the first device and a control grid, means for impressing a second bias of a different magnitude than said first bias on said last-named control grid, means common to both said grids for impressing said control potential on both said grids and a relay having its coil in series with the space discharge path of said second device, the contacts of the last-named connected in series with the contacts of the relay in the discharge path of the first device, one of said pairs of contacts being normally open and the other normally closed.

2. An electronic control circuit having an output stage comprising a first grid controlled discharge device, means for of said first device a first biasing potential, and a first relay having its actuating mechanism connected in the discharge path of said device and its contacts in series with a controlled circuit, the improvement which comprises a second grid controlled discharge device in parallel with the first discharge device, means for applying to the grid of said second discharge device a second biasing potential substantially lower than said first potential, a second relay having an actuating mechanism in series with the discharge path of the second discharge device, contacts of said second relay being connected in series with the applying to the grid both said devices for impressing a control potential on the grids of both said devices.

3. An electronic control circuit having an output stage comprising a first grid controlled discharge device, means for applying to the grid of charge path of said device and a pair of normally series with a controlled circuit, which comprises a second grid controlled discharge device in parallel with the first discharge device, means for applying to the grid of said second discharge device a biasing potential substantially less than the biasing pcon them.

4. In an electronic control circuit capable of delivering a control potential in accordance with the condition being sensed, a control circuit comprising a first discharge device having an anode, a cathode and a control grid, means for impressing a first biasing potential on said control coil in series with References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,128,081 Vogel Feb. 9, 1915 2,001,836 Craig May 21, 1935 2,150,265 Conover Mar. 14, 1939 2,225,346 Pierre Dec. 17, 1940 2,406,856 Satterlee Sept. 3, 1946 2,428,702 Elliot Oct. 7, 1947 2,509,497 Hesson May 30, 1950 

